Download Result of microbial growth in milk

Microbiology of Milk
SABILLO, Janneil Marie
PUSO, Marie Hecelle Antonette
HESNI, Shadi
Microbiology and Deterioration
of Milk and Milk Products
• Milk constitutes an excellent medium for the
growth of micro-organisms.
• Freshly drawn milk from healthy animals
contains a small number of harmless microorganisms, however, during milking process and
storage the contamination takes place, the extent
of which depends upon the hygienic measures
taken before, during and after milking process
and storage conditions observed thereafter.
 Nearly all the changes that take place in the
flavor and appearance of the milk after it is
drawn form the cow, are the result of the
activities of microorganisms ,therefore, it is
very
essential
to
control
these
microorganisms.
Significance of
Microorganisms in Milk
Microbial content serves as an indicator of
production conditions and sanitary quality
of milk.
Prevention of spoilage.
 Prevention of milk borne illnesses.
Production of dairy products with desired
characteristics imparted via m.o.
introduction
Microorganisms in Milk
The types of microorganisms found in milk vary
considerably
 Bacteria, yeasts, moulds and
bacteriophages are commonly encountered.
 Viruses and protozoa are seldom observed in
milk, except as occasional contaminants.
*Bacteria
Most common and most numerous of
microorganisms found in milk and milk products
.

1.
2.
3.
4.
They belong to four main groups:
Gram +ive cocci
Gram +ive non-spore forming rods
Gram +ive spore-forming rods
Gram -ive non-spore forming rods.
Normally present in milk and they are also
used as starter culture for the production of
cultured dairy products. They ferment lactose
and yield lactic acid.
i.
•
•
ii.
•
•
•
iii.
Lactococci
L.delbrueckii sub sps.lactis (Str.Lactis)
L. Lactis sub sps. Cremoris (Str. Cremoris)
Lactobacilli
L. Casei
L. delbrueckii sub.sps.lactis (L. Lactis).
L. delbrueckii sub Sps. bulgaricus
(L.bulgaricus)
Leuconostoc
Coliforms
• Facultative anaerobes
• Optimum growth @ 370c
• Indicator organism and are closely associated
the presence of pathogens but not necessary
pathogens
themselves.
• They ferment lactose with production of acid
and gas
• cause rapid spoilage of milk
• they are killed by pasteurization
• Their presence is an indication of PP
contamination of milk.
Yeast
 Most frequently encountered in raw cream during
hot weather produce acid and co2. They are
potential contaminants throughout the year.
Moulds
 Their growth is visible as a fuzzy or fluffy growth
on the surface of milk and milk products.
 They may be black, green, grey, blue or white.
 They discolor the product and often produce
repulsive undesirable off odors
Essential in production of certain varieties of
cheese.
Bacteriophages
 Particularly obnoxious in starter cultures used
for the production of cultured dairy products.
 Phages kill bacterial culture and entire
fermentation process fails (slow or dead vat).
Factors affecting growth of
Microorganisms in Milk
1. Food supply(H2o,energy,C,N Vit.
&Mineral source)
2. Moisture
3. Oxygen supply (Obligate aerobes
,facultative ,micoaerophillic ,
Aerotolerant anaerobes,obligate
anaerobes)
4. Acidity and pH (Acidophilic)
5.
6.
7.
8.
Preservatives
Light ( phototrophic)
Concentration (osmophillic yeasts)
Temperature (psychrotrophs- 20-30-07 ,
Mesophiles-30-40,Thermophiles-55-65o c
9. Antimicrobial constituents
Products of Microbial Growth in Milk
~Enzymes
~ Decomposition products of protein, fat
& CHO etc.
~ Pigments , Toxins etc (mycotoxins &
bacterial toxin )
Result of microbial growth in milk (Spoilage)
o Principal cause are Psychrotrophs
o Most of these are destroyed by pasteurization
o Some may survive e.g. Pseudomonas fluorescens ,
Pseudomonas fragi
o Other species and strains that survive pasteurization and
grow at Refrigeration temp. ,Produce heat stable
proteolytic & lipolytic enzymes and cause spoilage :
~Bacillus ~ Clostridium ~ Cornebacterium ~Arthrobacter ~
Lactobacillus ~ Microbacterium
~ Streptococcus
~ Micococcus
Deteriorative changes
 Souring- Lactose fermentation
LA, VFA
 Souring & gassiness- coliforms acid & gas
 Aroma production- starter culture  diacetyl
 Proteolysis- unpleasant odors- undesirable.
controlled desirable –cheese production.
 Ropiness- Milk drawn into long threads
(Alkaligenes viscolactis)
 Sweet curdling- due to prodn of rennin like enzyme
which curdles without souring
 Stormy fermentation- Rapid fermentation by
Clostridium perfringens
Color changes
Pseud.syncyanea(blue);Pseud.synxantha(yellow);
Serratia marcescens (red)
Stages of milk decay
Rancid
(“on the turn”)-Milk consumable
Curdling
Separation of curd and whey – milk
still consumable
Coagulation
period of aromatic decay with mould
growth –milk beyond use
Dry
dehydration ensues- hard and chalky
–milk beyond use
Pathogenic Microorganisms in Milk
•
Food borne illnesses occur as a result of :
~Ingestion of raw milk
~Improper pasteurization
~Poor handling / storage leading to PP contamination
• Measures to decrease the threat :
~Hygienic production practices
~Proper handling and storage
~Pasteurization
Microbial Pathogens
Bacillus cereus
Listeria monocytogens
Yersinea entrocolitica
Salmonella spp.
E.coli O 157:H7
Compylobacter jejuni
Coxeilla burnetii
Moulds(Aspergillus,Fusarium,Penicillium) grow in milk
and milk products & produce potentially
hazardous mycotoxins.
Control and Practical Application
1. Heat - pasteurization , sterilization etc.
2. Ionizing Radiations- UV., gamma rays
3. High frequency sound waves- super &
ultrasonic
4. Electricity - by the heat generated.
5. Pressure - 600 x > atmospheric pressure
6. Chemicals Acids ,alkalis, halogens H2 02 etc.